I just bought me a Motif XS 6 yesterday. I've always been a motif dude so I'm pretty familiar with it. Some of the sounds that are "church ready" are Full Concert Grand(its the first voice in the acoustic piano list when you push catagory search). Another one I like to use is the Soft R&B rhodes and the neo-soul rhodes sound. If you want to get some serious worship or warfare going then use the piano and strings sound(in the ES you have to go thru the performance section of the keyboard to get to it but in the XS its under the acoustic piano section...just look for it, you'll find it). Let me know if you have anymore questions.
On the ES8, if you want a piano and string combo, use the patch called Pianoback, and use the 1st and 3rd sliders to adjust the amount of piano or string. Its near the botom of the list in the piano category.
I also like the "rnb soft" and "neo soul" electric piano patches. for regular pianos use the concert grand.
For organ sounds, I use "fully" the fist slider is your 16',8'+4' drawbar. the second slider is your 5' 1/3 drawbar. the 3rd one is for your 2' +1' drawbars and the 4th one is your last two black dissonant drawbars.
Here is how to save sounds in your favorites
For basic playing, just press VOICE...CATEGORY SEARCH...and the voice you wanna use. To save voices in your favorites, just choose the voice, then press F5, you will see an X in the box next to the voice, then press the favorites button, all your selections will be in there.
Here is a thread with a lot of info on the Mo8/motif
http://www.learngospelmusic.com/forums/index.php/topic,53381.0.html